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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation. Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.lang;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.BiFunction;

/**
 * A WeakHashMap-like data structure that uses a pair of weakly-referenced keys
 * with identity equality semantics to associate a strongly-referenced value.
 * Unlike WeakHashMap, this data structure is thread-safe.
 *
 * @param <K1> the type of 1st key in key pair
 * @param <K2> the type of 2nd key in key pair
 * @param <V> the type of value
 * @author Peter Levart
 */
final class WeakPairMap<K1, K2, V> {

 private final ConcurrentHashMap<Pair<K1, K2>, V> map = new ConcurrentHashMap<>();
 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

 /**
 * Tests if the specified pair of keys are associated with a value
 * in the WeakPairMap.
 *
 * @param k1 the 1st of the pair of keys
 * @param k2 the 2nd of the pair of keys
 * @return true if and only if the specified key pair is in this WeakPairMap,
 * as determined by the identity comparison; false otherwise
 * @throws NullPointerException if any of the specified keys is null
 */
 public boolean containsKeyPair(K1 k1, K2 k2) {
 expungeStaleAssociations();
 return map.containsKey(Pair.lookup(k1, k2));
 }

 /**
 * Returns the value to which the specified pair of keys is mapped, or null
 * if this WeakPairMap contains no mapping for the key pair.
 * <p>More formally, if this WeakPairMap contains a mapping from a key pair
 * {@code (_k1, _k2)} to a value {@code v} such that
 * {@code k1 == _k1 && k2 == _k2}, then this method returns {@code v};
 * otherwise it returns {@code null}.
 * (There can be at most one such mapping.)
 *
 * @param k1 the 1st of the pair of keys for which the mapped value is to
 * be returned
 * @param k2 the 2nd of the pair of keys for which the mapped value is to
 * be returned
 * @return the value to which the specified key pair is mapped, or null if
 * this map contains no mapping for the key pair
 * @throws NullPointerException if any of the specified keys is null
 */
 public V get(K1 k1, K2 k2) {
 expungeStaleAssociations();
 return map.get(Pair.lookup(k1, k2));
 }

 /**
 * Maps the specified key pair to the specified value in this WeakPairMap.
 * Neither the keys nor the value can be null.
 * <p>The value can be retrieved by calling the {@link #get} method
 * with the same keys (compared by identity).
 *
 * @param k1 the 1st of the pair of keys with which the specified value is to
 * be associated
 * @param k2 the 2nd of the pair of keys with which the specified value is to
 * be associated
 * @param v value to be associated with the specified key pair
 * @return the previous value associated with key pair, or {@code null} if
 * there was no mapping for key pair
 * @throws NullPointerException if any of the specified keys or value is null
 */
 public V put(K1 k1, K2 k2, V v) {
 expungeStaleAssociations();
 return map.put(Pair.weak(k1, k2, queue), v);
 }

 /**
 * If the specified key pair is not already associated with a value,
 * associates it with the given value and returns {@code null}, else does
 * nothing and returns the currently associated value.
 *
 * @param k1 the 1st of the pair of keys with which the specified value is to
 * be associated
 * @param k2 the 2nd of the pair of keys with which the specified value is to
 * be associated
 * @param v value to be associated with the specified key pair
 * @return the previous value associated with key pair, or {@code null} if
 * there was no mapping for key pair
 * @throws NullPointerException if any of the specified keys or value is null
 */
 public V putIfAbsent(K1 k1, K2 k2, V v) {
 expungeStaleAssociations();
 return map.putIfAbsent(Pair.weak(k1, k2, queue), v);
 }

 /**
 * If the specified key pair is not already associated with a value,
 * attempts to compute its value using the given mapping function
 * and enters it into this WeakPairMap unless {@code null}. The entire
 * method invocation is performed atomically, so the function is
 * applied at most once per key pair. Some attempted update operations
 * on this WeakPairMap by other threads may be blocked while computation
 * is in progress, so the computation should be short and simple,
 * and must not attempt to update any other mappings of this WeakPairMap.
 *
 * @param k1 the 1st of the pair of keys with which the
 * computed value is to be associated
 * @param k2 the 2nd of the pair of keys with which the
 * computed value is to be associated
 * @param mappingFunction the function to compute a value
 * @return the current (existing or computed) value associated with
 * the specified key pair, or null if the computed value is null
 * @throws NullPointerException if any of the specified keys or
 * mappingFunction is null
 * @throws IllegalStateException if the computation detectably
 * attempts a recursive update to this map
 * that would otherwise never complete
 * @throws RuntimeException or Error if the mappingFunction does so, in
 * which case the mapping is left unestablished
 */
 public V computeIfAbsent(K1 k1, K2 k2,
 BiFunction<? super K1, ? super K2, ? extends V>
 mappingFunction) {
 expungeStaleAssociations();
 try {
 return map.computeIfAbsent(
 Pair.weak(k1, k2, queue),
 pair -> mappingFunction.apply(pair.first(), pair.second()));
 } finally {
 Reference.reachabilityFence(k1);
 Reference.reachabilityFence(k2);
 }
 }

 /**
 * Returns a {@link Collection} view of the values contained in this
 * WeakPairMap. The collection is backed by the WeakPairMap, so changes to
 * the map are reflected in the collection, and vice-versa. The collection
 * supports element removal, which removes the corresponding
 * mapping from this map, via the {@code Iterator.remove},
 * {@code Collection.remove}, {@code removeAll},
 * {@code retainAll}, and {@code clear} operations. It does not
 * support the {@code add} or {@code addAll} operations.
 *
 * @return the collection view
 */
 public Collection<V> values() {
 expungeStaleAssociations();
 return map.values();
 }

 /**
 * Removes associations from this WeakPairMap for which at least one of the
 * keys in key pair has been found weakly-reachable and corresponding
 * WeakRefPeer(s) enqueued. Called as part of each public operation.
 */
 private void expungeStaleAssociations() {
 WeakRefPeer<?> peer;
 while ((peer = (WeakRefPeer<?>) queue.poll()) != null) {
 map.remove(peer.weakPair());
 }
 }

 /**
 * Common interface of both {@link Weak} and {@link Lookup} key pairs.
 */
 private interface Pair<K1, K2> {

 static <K1, K2> Pair<K1, K2> weak(K1 k1, K2 k2,
 ReferenceQueue<Object> queue) {
 return new Weak<>(k1, k2, queue);
 }

 static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) {
 return new Lookup<>(k1, k2);
 }

 /**
 * @return The 1st of the pair of keys (may be null for {@link Weak}
 * when it gets cleared)
 */
 K1 first();

 /**
 * @return The 2nd of the pair of keys (may be null for {@link Weak}
 * when it gets cleared)
 */
 K2 second();

 static int hashCode(Object first, Object second) {
 // assert first != null && second != null;
 return System.identityHashCode(first) ^
 System.identityHashCode(second);
 }

 static boolean equals(Object first, Object second, Pair<?, ?> p) {
 return first != null && second != null &&
 first == p.first() && second == p.second();
 }

 /**
 * A Pair where both keys are weakly-referenced.
 * It is composed of two instances of {@link WeakRefPeer}s:
 * <pre>{@code
 *
 * +-referent-> [K1] +-referent-> [K2]
 * | |
 * +----------------+ +----------------+
 * | Pair.Weak <: |-----peer----->| (anonymous) <: |
 * | WeakRefPeer, | | WeakRefPeer |
 * | Pair |<--weakPair()--| |
 * +----------------+ +----------------+
 * | ^
 * | |
 * +-weakPair()-+
 *
 * }</pre>
 * <p>
 * Pair.Weak is used for CHM keys. Both peers are associated with the
 * same {@link ReferenceQueue} so when either of their referents
 * becomes weakly-reachable, the corresponding entries can be
 * {@link #expungeStaleAssociations() expunged} from the map.
 */
 final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> {

 // saved hash so it can be retrieved after the reference is cleared
 private final int hash;
 // link to <K2> peer
 private final WeakRefPeer<K2> peer;

 Weak(K1 k1, K2 k2, ReferenceQueue<Object> queue) {
 super(k1, queue);
 hash = Pair.hashCode(k1, k2);
 peer = new WeakRefPeer<>(k2, queue) {
 // link back to <K1> peer
 @Override
 Weak<?, ?> weakPair() { return Weak.this; }
 };
 }

 @Override
 Weak<?, ?> weakPair() {
 return this;
 }

 @Override
 public K1 first() {
 return get();
 }

 @Override
 public K2 second() {
 return peer.get();
 }

 @Override
 public int hashCode() {
 return hash;
 }

 @Override
 public boolean equals(Object obj) {
 return this == obj ||
 (obj instanceof Pair &&
 Pair.equals(first(), second(), (Pair<?, ?>) obj));
 }
 }

 /**
 * Optimized lookup Pair, used as lookup key in methods like
 * {@link java.util.Map#get(Object)} or
 * {@link java.util.Map#containsKey(Object)}) where
 * there is a great chance its allocation is eliminated
 * by escape analysis when such lookups are inlined by JIT.
 * All its methods are purposely designed so that 'this' is never
 * passed to any other method or used as identity.
 */
 final class Lookup<K1, K2> implements Pair<K1, K2> {
 private final K1 k1;
 private final K2 k2;

 Lookup(K1 k1, K2 k2) {
 this.k1 = Objects.requireNonNull(k1);
 this.k2 = Objects.requireNonNull(k2);
 }

 @Override
 public K1 first() {
 return k1;
 }

 @Override
 public K2 second() {
 return k2;
 }

 @Override
 public int hashCode() {
 return Pair.hashCode(k1, k2);
 }

 @Override
 public boolean equals(Object obj) {
 return obj instanceof Pair &&
 Pair.equals(k1, k2, (Pair<?, ?>) obj);
 }
 }
 }

 /**
 * Common abstract supertype of a pair of WeakReference peers.
 */
 private static abstract class WeakRefPeer<K> extends WeakReference<K> {

 WeakRefPeer(K k, ReferenceQueue<Object> queue) {
 super(Objects.requireNonNull(k), queue);
 }

 /**
 * @return the {@link Pair.Weak} side of the pair of peers.
 */
 abstract Pair.Weak<?, ?> weakPair();
 }
}